Elimination of chemicals
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elimination of chemicals, mechanism to get rid of drugs and chemicals (see also PCB - elimination, PCDD/F - elimination). Elimination out of the body takes place in two principal ways: excretion and metabolism (usually followed by excretion of the breakdown products). Only water-soluble materials can be excreted in the kidneys to urine, and many organic pollutants are lipid soluble and poorly water-soluble chemicals. Therefore they cannot be excreted practically at all as such. Metabolism tries to make them more water soluble, but especially higher chlorinated PCBs (see PCB - physicochemical properties) and PCDD/Fs with "lateral" chlorine atoms (see this and PCDD - chemical structure, PCDF - chemical structure) are metabolised very poorly, and therefore cannot be effectively excreted even with the help of metabolism. They accumulate in body fats, and their half-life (see this) may be even several years. Elimination of drugs and chemicals usually obeys first order kinetics (see half-life, cumulation). This means that the rate of elimination directly correlates with the amount of the drug in the body (or the concentration in blood), i.e. a constant fraction (e.g. one per cent) of the chemical is eliminated in time unit (e.g. in an hour).